1. Field of the Invention
This invention relates to systems used to determine the orientation of a geosynchronous orbiting satellite relative to the earth. More specifically, this invention relates to satellite orientation systems which utilize reference signals generated on earth.
While the present invention is described herein with reference to a particular embodiment, it is understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional embodiments within the scope thereof.
2. Description of the Related Art
Stabilization of spacecraft in geosynchronous earth orbit has typically been effected by spin stabilization or 3-axis stabilization. In the former approach the craft is induced to spin at launch about a longitudinal axis oriented in a desired direction relative to the earth. However, the instruments deployed on spin-stabilized craft are only trained on the earth during a fraction of each rotation. Consequently, the radiometric resolution of images obtained by these on-board instruments is less than could be achieved were they to be continuously pointed in the direction of the earth. In the 3-axis stabilization approach, the spacecraft or satellite is maintained in a fixed orientation by momentum wheels spinning about the three orientation axes of the craft. A higher duty-cycle is afforded the on-board sensors of 3-axis stabilized satellites compared to those on spin-stabilized satellites.
However, spin-stabilized satellites inherently provide greater orientation stability than those employing 3-axis stabilization. Successful exploitation of the improved instrument performance offered by 3-axis stabilized satellites thus hinges on the provision of precision pointing and angular orientation control. Such control is typically implemented by providing orientation error information to a servo system operative to adjust the alignment of the satellite.
At least two methods currently exist for obtaining error information relating to the orientation of a satellite. In a first method, data from star sensors in combination with orbital parameters provided by an earth-based tracking station furnish reference information for a navigational unit deployed on-board the satellite. In particular, the on-board unit is operative to detect perturbations in a desired orbital pattern once it has been supplied information relating to the orientation and position of the satellite at a reference time. Typical on-board units include either an inertial monitoring unit (IMU), an angular displacement sensor (ADS), or a combination of the two.
One disadvantage of the star-referenced method for obtaining orientation error information is that momentary "interrupts" of spacecraft electronics require that the relative location of the reference star or constellation again be determined in order to appropriately calibrate the on-board unit. The satellite may thus be pointed and angularly oriented incorrectly for significant time periods while the position of a reference star is attempted to be ascertained.
In a second method for sensing the deviation of a satellite from a desired orientation, a multiplicity of distinct earth features are detected by an on-board staring infrared sensor. Information from the sensor is then processed in order to generate an error signal indicative of the satellite orientation. A drawback inherent in the second method is that certain weather conditions can obscure the geological earth features necessary to determine proper alignment of the satellite. Accordingly, control of the pointing and angular orientation of the spacecraft is susceptible to interruption.
Hence, a need in the art exists for a system capable of providing satellite orientation information despite the existence of an atmospheric disturbance or the interruption of on-board electronics.